1. Field of the Invention
This invention relates generally to magnetic read and write-heads and more particularly to a method of eliminating electrostatic discharge (ESD), cross-talk and noise during their operation.
2. Description of the Related Art
A magnetic data recording hard disk drive employs a plurality of electromagnetic transducers, a typical one of the prior art being schematically shown in FIG. 1. In a state-of-the-art disk drive, each transducer (1) further consists of a read-head (2) and a write-head (4). The shielded read head (2) typically includes a sensor (13) whose operation is based on the giant magnetoresistive (GMR) effect and it is extremely sensitive to electrostatic discharge (ESD). As we shall discuss below, three different configurations of sensors are in use, the current-in-plane (CIP) configuration, the current-perpendicular-to-plane (CPP) configuration and the tunneling magnetoresistive (TMR) configuration.
The write-head (4) is typically an inductive coil (coil cross-sections shown as (16)), which is fairly immune to ESD. The read head is typically protected from stray electromagnetic fields during operation by an upper shield (8) and an under-shield (10). The lower pole of the write head (18) can also serve to insulate the read head from noise produced by the write head. If the read head is a CIP head, a first dielectric layer, D1 (11) insulates the under-shield from the CIP element (13), and second dielectric layer, D2 (15) insulates the element from the upper shield (8). In the CIP configuration, the sense current for the read head is introduced on the lateral sides of the read head. In the CPP or TMR configurations, in both of which the sense current is introduced through the top and bottom surfaces, the shields may be formed directly contacting the upper and lower surfaces of the element and may actually serve as electrodes to introduce and extract the sense current. The dielectric layers D1 and D2 would be absent in the CPP or TMR configuration.
The shields, dielectric layers and GMR sensor structure are sequentially formed on a substrate (20), also called a slider substrate. The combined read/write-head (transducer) is typically encapsulated in an alumina deposit (12) and conducting leads (not shown) pass through the encapsulation and are connected to conducting pads on a terminal strip (14) on its surface. An undercoating layer of alumina (5) separates the lower surface of the lower shield (10) from the substrate (20) and a dielectric spacer layer typically formed of the alumina encapsuation material (6) separates the upper surface of the upper shield (8) from the lower surface of the write head pole (18). In the present invention the thicknesses of these layers will be important in determining coupling capacitances between the shields and the substrate.
The slider substrate (20) basically forms a wear-resistant ceramic carrier for the transducers (1). The write head's magnetic poles (18) and the read head's GMR sensor element (13), emerge at a surface of their encapsulating alumina deposit (12) which is co-planar with the surface (22) of the slider substrate, called its air-bearing surface (ABS). The surface of the slider is commonly protected by a highly wear-resistant carbon overcoat (COC) (21). As is shown further, each slider substrate (20) is mounted on a stainless steel suspension (23) using a conductive adhesive (24), the combination of (20) and (23) forming a head-gimbals assembly (HGA). The conducting adhesive electrically connects the slider to the suspension and to ground.
The read head ((2) in FIG. 1) may be formed in any of several GMR configurations. A common configuration is the current-in-plane (CIP) configuration, in which the sense current is injected at lateral edges of the sensor and flows along the direction of the planar surfaces of the magnetic layers. Another configuration is the current-perpendicular-to-plane (CPP) configuration, in which the sense current is injected through the upper surface of the sensor in a direction perpendicular to the planar surfaces of the magnetic layers. Yet another read head variation is the tunneling magnetoresistive (TMR) device, in which the active magnetic layers are separated by a dielectric layer sufficiently thin so as to allow a tunneling current to pass.
GMR devices are susceptible to noise, cross-talk from the write head and electrostatic discharge damage (ESD). The cross-talk is a result of the closeness of the write and read heads and the ESD damage is a result of the thinness of the read head layers. To protect the device from such discharge, a mechanism is required that will bleed accumulated charge from the sensor and its surrounding elements before the accumulated amount is sufficient to initiate the discharge. Referring to prior art FIG. 2, there is shown a schematic illustration of an ABS plane view of a GMR sensor of CIP configuration (13) positioned between an upper shield (8) and a lower shield (10) and separated from the slider substrate (20) by insulation (12). Rectangular boxes (50, 55) symbolize sensor current input and output to the lateral edges of the sensor in accord with its CIP configuration. A lower pole (18) of an inductive write head is shown above the upper shield and is also insulated from the shield by a dielectric layer (6). A connection (30) between the lower pole and upper shield allows charge drainage from pole to shield. A connection (40) from lower shield to substrate, which is itself grounded to the suspension (not shown), contains a resistance of approximately 20 kOhm. The upper shield is directly connected to the lower shields which are connected to each side of the CIP sensor through resistances (60,65) of approximately 2 kOhm each. The shields can discharge to the substrate or the suspension or when connected to the read head preamplifier. This configuration is found to provide adequate ESD protection to the CIP GMR sensor as shown.
CPP GMR sensors and TMR sensors also need a current drainage path to prevent ESD, but the arrangement of FIG. 2 would not be appropriate. If the sensor (13) of FIG. 2 were a CPP configured sensor, the upper and lower shields (8,10) would actually be in electrical contact with the current input (and output) ends of the sensor, which are its upper and lower surfaces. Thus, the two resistors (60,65) would be in parallel with the actual read element, whose resistance ranges from 50 Ohms (CPP) to 1000 Ohms (TMR). This would either force most of the draining charges through the sensor, or would shunt the read-back signal from the sensor through the resistors and away from the read-back circuitry. Neither of these are desirable results.
Referring to prior art FIG. 3, there is shown the equivalent schematic circuitry associated with a CPP GMR or TMR type read head. There is no ESD preventative circuitry shown in this figure because, as noted above, circuitry analogous to that in FIG. 2 would be inappropriate. The sensor element (13) is between upper and lower shields (8, 10) which receive sense current from input and output (50,55), the current passing vertically between upper and lower shields. As is further indicated in the figure, capacitative coupling (28) exists between the lower write head pole (18), the upper and lower shields (8,10) and the slider substrate (20), which produces an undesirable degree of cross-talk between the write head and the read head and between the substrate and read head. This capacitative coupling is a result of the planar shields, pole and the dielectric insulative material between them and the slider substrate. This capacitative coupling is much less of a problem for the CIP configuration of FIG. 3 because of the symmetric and balanced configuration of the CIP leads. In the CPP configuration the capacitance between the pole and the first lead is different than that between the pole and the second lead. Similarly, the capacitance between the substrate and the first lead is different than that between the substrate and the second lead. In the CPP structure all the capacitances to the first and second leads are similar due to the way the structure is laid out.
The prior art teaches several approaches for eliminating ESD in both disk drives and tape drives that incorporate GMR and MR read sensors. Of particular note is Tabat et al. (U.S. Pat. No. 6,728,082) who teaches a magnetoresistive transducer that includes at least one bleed resistor that couples the assembly to a substrate. While Tabat does not specifically note the GMR configuration of the sensor, he shows a sensor shield connected by a high resistance to a substrate. Tabat does not mention the associated problem of capacitative coupling induced noise and cross-talk between the read head and write head, although he notes the fact that the read head shields couple capacitatively to the substrate. Neither does Tabat teach a shunting scheme that eliminates noise and cross-talk caused by capacitative coupling between the read head and write head at the same time it eliminates ESD.
Lam et al. (U.S. Pat. No. 6,373,660) teach a method for protecting an MR read head from electrostatic discharge by connecting a resistor between various connection pads incorporated within the head gimbal assembly.
Denison et al. (U.S. Pat. No. 5,539,598) discloses a magnetoresistive head for reading a tape that is protected from ESD by a resistive connection between a shield and a ground. Unlike the present invention, there is no associated write head and no capacitative coupling between the head and its surroundings.
Yuan et al. (U.S. Pat. No. 6,219,205) teaches the protection of a read head sensor by recessing it from the surface to be read by a dielectric layer. The method is, therefore, mechanical rather than electrical.
Unlike the prior art cited above, the present invention proposes a resistive/capacitative shunting scheme for slowly bleeding off accumulated electrostatic charge on shields surrounding a CPP GMR and TMR sensor and from the pole structures of an adjacent magnetic write head. In addition, by combining shunting resistors with a proper choice of capacitative coupling between the shields, pole piece and substrate, the combined resistive/capacitative shunting scheme will also alleviate the problem of noise and cross-talk produced by capacitative coupling between the shields, pole and substrate that otherwise exists without the shunting scheme of the present invention.